172 IRON is dissolved in water and the solution transferred to a porcelain dish and acidified with hydrochloric acid. After evaporation to dryness on a steam- bath, the residue is moistened with hydrochloric, acid and again evaporated, heated in an oven at 135°, taken np with hydrochloric acid, diluted, heated and, after cooling, filtered ; the silica is subsequently washed, dried, ignited in a platinum crucible and weighed. 2. Attack of the Metal with Nitric Acid. With grey cast-iron or silicon steel (3-5% Si), 2 4 gnims, or with white cast iron or ordinary steel, 5-1C) grams, of the. sample1, are introduced into a thin porcelain dish 12-15 cm. in diameter. The dish is covered with a clock-glass and dilute nitric acid (1) ;ri8) gradually added, addition of fresh acid being made only when the initial vigorous action begins to slacken. Kaeh gram of metal requires about 12 c.c. of acid. When the required amount of acid has been added, the dish is heated on a steam-bath to complete the action l ; the clock-,1.;lass is then removed and washed and the liquid evaporated on a. steam-bath, the dish being frequently shaken to break the skin forming on the surface, and towards the end of the operation the liquid stirred with a platinum spatula to pre- vent spurting, When dry, the residue is healed further on a, sand-bath or asbestos until a, powdery residue remains ; the heat is then gradually increased to redness, which is maintained until the nitrates are completely decomposed, i.e., until evolution of brown vapours ceases. The cold residue, is moistened with cone, hydrochloric, acid and heated lor a short time, a further addition of 9-10 c.c. of cone, hyth'ochloric acid per gram of metal dissolved being made and (lie liquid heated and stirred until the. ferric oxide is completely dissolved. To ensure that the silica becomes absolutely insoluble, the hydrochloric acid solution is evaporated to dryness and the, residue heated lor an hour in an oven at 135". The residue is taken up again in cone, hydrochloric acid (5- (> c.c. oC acid per gram of metal), heated, diluted and, when cold, filtered, the silica being purified in the same way as when the metal is attacked with hydro- chloric acid (sw p, 171). The filtrate may be used for the determination of phosphorus (sec> p. 173). 3, Determination of Manganese Manganese, which is always present in larger or smaller amount; in iron, cast-iron or steel, may be estimated graviineldeally, volume!rieally or eolorimetrieally. The volumetric method will be described later (.w Ferro- manganese) and here will be given only the colonmetrie method, which, owing to its rapidity, is the most commonly used for determining the small proportions of manganese contained in iron and in ordinary steels. Golorimctrlc Determination of Manganese (according to Ledebur). ......-This method consists in acting on the sample with nitric acid, oxidising 1 The, inure or lews intensely In-own liquid should, ho\vrvt:i% In- clt'ur ; if it appears turbid, u little more acid .should be udded.on of silicon in products insoluble in acid, see Ferro-silicon.